As the development of nanoscale mechanical, electrical, chemical and biological devices and systems increases, new processes and materials are needed to fabricate nanoscale devices and components. Conventional optical lithographic processing methods are not able to accommodate fabrication of structures and features much below the 100 nm level. The use of self-assembling diblock copolymers presents another route to patterning at nanometer dimensions. Diblock copolymer films spontaneously assembly into periodic structures by microphase separation of the constituent polymer blocks after annealing, for example by thermal annealing above the glass transition temperature of the polymer or by solvent annealing, forming ordered domains at nanometer-scale dimensions. Following self-assembly, one block of the copolymer can be selectively removed and the remaining patterned film used as an etch mask for patterning nanosized features into the underlying substrate. Since the domain sizes and periods (Lo) involved in this method are determined by the chain length of a block copolymer (MW), resolution can exceed other techniques such as conventional photolithography, while the cost of the technique is far less than electron beam lithography or EUV photolithography, which have comparable resolution.
The film morphology, including the size and shape of the microphase-separated domains, can be controlled by the molecular weight and volume fraction of the AB blocks of a diblock copolymer to produce lamellar, cylindrical, or spherical morphologies, among others. For example, for volume fractions at ratios greater than about 80:20 of the two blocks (AB) of a diblock polymer, a block copolymer film will microphase separate and self-assemble into a periodic spherical domains with spheres of polymer B surrounded by a matrix of polymer A. For ratios of the two blocks between about 60:40 and 80:20, the diblock copolymer assembles into a periodic hexagonal close-packed or honeycomb array of cylinders of polymer B within a matrix of polymer A. For ratios between about 50:50 and 60:40, lamellar domains or alternating stripes of the blocks are formed. Domain size typically ranges from 5-50 nm.
Diblock copolymer thin films of cylindrical and lamellar phases may both form striped phases relative to an interface. For cylindrical phase films, a striped pattern results from parallel cylinder orientation, while for lamellar phase films, a striped pattern results from perpendicular domain orientation. From a top down view, perpendicular-oriented lamellae and parallel-oriented cylinders appear similar, e.g., as parallel lines.
Graphoepitaxy techniques using defined topography such as trench edges have been used in an attempt to orient and order copolymer domains and control registration and alignment of the self-assembled blocks to form a desired pattern.
While the self-assembly of diblock copolymers of tightly controlled composition and polydispersity has been demonstrated as a method of preparing a variety of nanoscale, sub-lithographic structures, the necessity of casting only very thin films limits the dimensions of the structures, particularly in the z-axis direction (i.e., perpendicular to the substrate surface). Openings produced by selective etching and removal of polymer domains of the films may not achieve the required aspect ratio for critical dimensions of desired features.
It would be useful to provide a method of fabricating films of arrays of ordered nanostructures that overcome these problems.